Field of the Invention
The present invention relates to a method for determining at least one measuring point-in-time in a cardiac cycle that can be used for conducting diffusion measurements of the myocardium of an examination object in a magnetic resonance (MR) system. The invention also relates to an associated MR system and to an electronically readable data carrier (storage medium).
Description of the Prior Art
Magnet resonance tomography (MRT) is a versatile imaging modality because MR images of an examination object can be generated with many different contrasts. The contrasts that are usually used are contrasts based on relaxation mechanisms, such as the T1 time or T2 time. Either native tissue properties or contrast media-induced properties of the tissue are used in this connection. Further contrast possibilities are the use of flow effects, such as the inflow of nuclear spins that have been given a magnetization into an imaging plane, the phase development of flowing nuclear spins, magnetization transfer methods, and diffusion.
Diffusion-weighted imaging has recently been applied in various regions of the body. With Diffusion Tensor Imaging (DTI) the variation in the direction of the measured diffusion of the water in the tissue is measured and analyzed. The observed variations in diffusion are generated by the movement of the water molecules in the tissue region in the spatial direction of the diffusion encoding. The spatial dependency of the diffusion depends on the geometry of the tissue region and can be described by a diffusion tensor. A set of diffusion measurements having different encoding directions is used for measuring the tensor properties. One possible application of DTI is known as “Fiber Tracking”, and this leads to a color depiction of the neuronal activity in the brain. DTI can be used in the heart to determine the geometry of the muscle cells in the myocardium. In a normal heart these cells are arranged in a helix structure, it being possible to depict the structure using DTI. The following documents are examples of this:
“In vivo measurement of water diffusion in the human heart,” Edelman R R, Gaa J, Wedeen V J, Loh E, Hare J M, Prasad P, Li W., Magn Reson Med. 1994 September; 32(3):423-8
“Cardiac diffusion tensor MRI in vivo without strain correction,” Tseng W Y, Reese T G, Weisskoff R M, Wedeen V J., Magn Reson Med. 1999 August; 42(2):393-403
“In vivo diffusion tensor MRI of the human heart: reproducibility of breath-hold and navigator-based approaches,” Nielles-Vallespin S, Mekkaoui C, Gatehouse P, Reese T G, Keegan J, Ferreira P F, Collins S, Speier P, Feiweier T, de Silva R, Jackowski M P, Pennell D J, Sosnovik D E, Firmin D., Magn Reson Med. 2013 August; 70(2):454-65
“Reproducibility of in-vivo diffusion tensor cardiovascular magnetic resonance in hypertrophic cardiomyopathy,” Laura-Ann McGill, Tevfik F Ismail, Sonia Nielles-Vallespin, Pedro Ferreira, Andrew D Scott, Michael Roughton, Philip J Kilner, S Yen Ho, Karen P McCarthy, Peter D Gatehouse, Ranil de Silva, Peter Speier, Thorsten Feiweier, Choukkri Mekkaoui, David E Sosnovik, Sanjay K Prasad, David N Firmin and Dudley J Pennell, Journal of Cardiovascular Magnetic Resonance 2012, 14:86
“Low b-value diffusion-weighted cardiac magnetic resonance imaging: initial results in humans using an optimal time-window imaging approach,” Rapacchi S, Wen H, Viallon M, Grenier D, Kellman P, Croisille P, Pai V M., Invest Radiol. 2011 December; 46(12):751
In the case of one of these methods, a periodic intensity modulation is encoded in one spatial direction during a first heartbeat. During the period of one heartbeat this modulation is stored as a longitudinal magnetization, which relaxes with the T1 time, with the diffusion occurring in this period of the heartbeat blurring the modulation pattern. After this encoding step of the modulation, the spatial modulation is decoded by reversing the modulation. Changes in the spatial modulation over the period due to the diffusion produce a signal attenuation. The movement of the heart will also influence the modulation. Decoding consequently occurs exactly in the same cardiac phase as the encoding in the previous heartbeat. The cardiac geometry and movement is therefore the same in the case of encoding and decoding, and this identifies the diffusion as the single attenuation mechanism in the blurring of the modulation pattern. The diffusion can thus be measured. Since the respiratory movement also influences the pattern, this influence is eliminated by measuring with breath-hold techniques.
Assuming that encoding takes place during diastole when the heart muscle is relaxed, the heart muscle contracts during the systole, and this leads to a change in the region geometry. For example, the thickness of the myocardium wall of the left ventricle increases in the radial direction during the contraction, and the modulation pattern is stretched in this direction. The diffusion that occurs in the contracted state in this direction has a smaller effect than the diffusion that occurs when the heart muscle is relaxed.
This means that the tissue formation relative to the geometry during the encoding time during the cardiac cycle influences the signal attenuation, and this leads to an error in the diffusion measurement. If the tissue is compressed the signal attenuation is intensified by diffusion, whereas the signal attenuation is reduced in one direction by diffusion if the tissue is pulled apart in this direction between encoding and decoding.
It is therefore necessary to reduce the influence of the compression or expansion in diffusion measurements. In order to obtain a diffusion measurement that is independent of the tissue deformation, it is therefore necessary to take into account the tissue deformation such as expansion or compression. This may be done in two ways:
1. The three-dimensional deformation pattern is measured during the cardiac cycle, and the measured attenuation corrected with the use of this data. A method of this kind is described in “Imaging myocardial fiber architecture in vivo with magnetic resonance,” Reese T G, Weisskoff R M, Smith R N, Rosen B R, Dinsmore R E, Wedeen V J. Magn Reson Med. 1995 December; 34(6):786-91.
Complex tagging methods or phase contrast measurements are required for measuring the deformation.
2. The distortion pattern is again measured during the cardiac cycle. The measurement is made in what is known as a sweet spot as the deformation effect is cancelled out. It was found in the document mentioned last that all deformation components vary roughly synchronously with time and that the diffusion varies roughly linearly with the deformation.
According to these requirements two sweet spots exist in a cardiac cycle. If the encoding and decoding of the diffusion are carried out so as to be coordinated with the point-in-times of the sweet spots then the measurement is independent of the deformation.
The methods mentioned above, however, are all complicated or time-consuming, however.